Antibodies to Polyomavirus BK (BKV) infection are seen in the majority of healthy adults. Reactivation of the virus occurs in immunosuppressed patients. In national databases, 4.6 to 9.2% of U.S. kidney transplants are complicated by BKV-nephropathy or BK viremia that requires therapeutic intervention. BKV also causes hemorrhagic cystitis which requires clinical intervention in about 5% of hematopoietic stem cell transplantation recipients. This grant will elucidate the humoral and cellular immune response to BKV infection. The work proposed work can be summarized in the form of 3 specific aims The first specific aim is to perform prospective longitudinal monitoring studies to determine if genotype- specific BKV neutralizing antibodies protect against BK viruria, viremia and nephropathy. Existing studies on the subject have been descriptive and not appropriately designed to determine if anti-BKV antibodies actually help in the resolution of infection. Furthermore, studies to date have not accounted for the genotype of the infecting strain. The latter consideration is important because BKV is known to have four major genotypes I, II, III, and IV. The second specific aim of the proposed work will be to determine the viral targets and cellular phenotypes of T-cells that mediate protective immunity against BKV. Blood samples collected during the course of a prospective longitudinal monitoring study will be used to characterize the cell-mediated immune responses to this virus. Patient lymphocytes will be stimulated by individual BKV genes or peptides. Changes in immunologic cell surface markers and intra-cellular cytokines will be measured. The third specific aim will determine the role of multiple infecting genotypes and quasispecies in the immunobiology of BKV infection. Recent work has shown that infection by multiple BK virus strains is common. Further studies are warranted to determine the biologic and clinical implications of this observation. Hence, this part of the project will seek to measure changes in BK virus load, viral genotype, genetic diversity, and quasispecies complexity that occur in conjunction with alterations in antibody titer and frequency of BKV sensitized T-cells. The study will determine if coinfection with multiple genotypes or mutant strains alters the clinical outcome. Relevance to Public Health: A study of the immune response to BKV infection is important to design active and passive antibody or cell based vaccines with the potential to benefit approximately 100,000 transplant patients annually. In addition, the knowledge generated by studying BKV will be relevant to understanding the immune response to several other polyomaviruses widely prevalent in man, including polyomaviruses JC, WI, and KU, Merkel cell virus, and Trichodysplasia spinulosa virus.